Nmtlp is an enzyme of 455 amino acids that catalyzes the transfer of myristate to the N-terminal glycine of cellular eukaryotic proteins. Myristoylation is used to mediate potentially reversible protein-protein and protein-membrane interactions that are necessary for the biological function of myristoylated proteins. N-myristoylation is an essential cellular process and proteins that become myrisloylated include protein tyrosine kinases, phosphatases, heterotrimeric G-proteins, as well as structural and non-structural proteins of numerous viruses including HIV. Nmts from pathogenic fungi are also important targets for design of antifungal drugs. We have recently solved the structure of Nmtlp in a ternary complex with a myristoylCoA and a peptide substrate analog using MAD data collected at BNL (Bhatnagar et al. (1998), Nature Structural Biology, in press). This ternary complex structure reveals the structural features that define the enzyme's substrate specificities and regulate the ordered binding and release of substrates and products. This structure also suggests a novel catalytic mechanism which involves deprotonation of the N-terminal ammonium of a peptide substrate by the enzyme's C-terminal backbone carboxylate. We now have crystals of the same enzyme bound to the same myristoylCoA analog but with one natural peptide substrate (crystal n 1). In addition, another ternary complex with a bound natural peptide inhibitor has been generated (crystal n 2). Solving the structure of a complex with a natural peptide will further delineate the novel mechanism of catalysis by this enzyme which we proposed based on the initial structure. Crystal n 1 and n 2 diffracted to 3.5 E and 3.3 E, respectively, were both in space group P3121 with unit cell dimensions a = b = 103 E, c = 108 E. Given the poor resolution of the data, synchrotron radiation would clearly be advantageous to this project.